1. Field of the Invention
The present invention relates generally to a force detecting device, and more particularly to a force detecting device known as a force transducer.
2. Discussion of Background
Some force detecting devices employ an elastic member called a strain element for detecting a force or moment applied to the elastic member based the elastic deformation of the elastic member. Such force detecting devices are known in respect of the wrist sensors of industrial robots, load measuring devices, steering devices, operating devices, and other devices. Various designs have been proposed for force detecting devices.
Forces or moments are electrically detected by force detecting devices. More specifically, a resistance strain sensor is attached to a surface of a strain element. When the strain element is elastically deformed under a force or moment applied, the strain sensor is also deformed with the strain element, changing its electric resistance. By electrically detecting such a change in the electric resistance, the amount of deformation of the strain element is determined, and hence the force or moment imposed on the strain element is known from the amount the strain element is deformed.
Conventional attempts to improve known force detecting devices or sensors are generally classified into two groups. In one group, efforts are made to increase the accuracy of detection. The other attempts are to facilitate the manufacture of the force detecting devices.
The detecting accuracy of a force detecting device is determined by the gage factor thereof. The gage factor G of a force sensor is defined by: ##EQU1## where R and L are the resistance and length, respectively, of the strain sensor of the force detecting device, and .DELTA.R is a change in the resistance which is caused when the length L is changed .DELTA.L. .DELTA.L/L here means a ratio of change, i.e., a strain (expressed by .epsilon.). For the strain sensors used in force detecting devices, .epsilon. varies in the range of 10.sup.-5 to 10.sup.-3. The gage factor G governs the relationship between the amount of deformation of the strain sensor and the change in the resistance which depends on the deformation. Naturally, the larger the gage factor G, the higher the force detecting accuracy becomes, or the more accurately the force can be detected. As the force detecting sensitivity is increased, the strain element may be of a more rigid and compact structure.
Japanese Laid-Open Patent Publications Nos. 58-118930, 59-75104, and 59-231431 disclose force detecting devices which employ a metal foil strain sensor as the strain sensor. The detecting accuracy of the metal foil strain sensor cannot be increased since the gage factor is of a quite low value such as 2 or 3 because the resistivity .rho. of metal is not varied by deformation.
Methods for manufacturing force detecting devices are shown in Japanese Utility Model Publications Nos. 54-11903, 54-21021 and Japanese Laid-Open Patent Publication No. 59-95433. According to the disclosed methods, a strain element and a strain sensor are separately fabricated, and the strain sensor is bonded to a surface of the strain element. However, it is tedious and time-consuming to bond and wire the strain sensor, with the result that the force detecting device may not necessarily be easy to manufacture. Another problem with these manufacturing methods is that the detecting accuracy varies from device to device because it depends on the accuracy of the position where the strain sensor is bonded, the type of the adhesive used, the wiring accuracy, and other factors.
One way of increasing the detecting accuracy of a strain sensor would be to construct a strain sensor of a single-crystal semiconductor. A strain sensor constructed of a single-crystal semiconductor would be of a high detecting accuracy since the gage factor G would be around 10.sup.2. However, the gage factor G of such a strain sensor would be highly dependent on temperature, thus requiring a temperature compensation circuit for stable detection of forces. Inasmuch as the gage factor is not linearly proportional to temperature, a linearizer would be needed for making the gage factor linearly dependent on temperature.